Very Low Intermediate Frequency (VLIF) receivers are popular due to their relatively low cost and small size, as well as their ability to operate over a broad range of frequencies. A VLIF receiver mixes a received radio frequency (RF) signal with a local oscillator prior to analogue to digital conversion. The local oscillator is set at a frequency close to that of the RF signal, but differing by an offset that is referred to as an intermediate frequency. For example, the intermediate frequency is typically between 3.6 kHz to 5.7 kHz, depending on channel spacing.
The local oscillator thus operates at a frequency either just above or just below the frequency of the RF signal, i.e., on one “side” of the RF signal. At times, this results in interference from an adjacent channel interferer on the side of the RF signal where the local oscillator is operating.
The down-mixed RF signal comprises an in-phase component, I, and a quadrature component, Q. Due to receiver components mismatch, there can be an amplitude and phase imbalance in otherwise perfectly balanced I and Q signals. An imbalance in the I and Q introduces an undesired spectral component referred to as the image signal that adversely impacts the receiver performance. In particular, when there is an adjacent channel interferer close to the desired signal of interest, the receiver performance can be severely degraded if the image of the adjacent channel signal falls within the frequencies occupied by the desired signal.
I/Q imbalance algorithms exist that attempt to estimate amplitude and phase imbalances in the RF signal and compensate for the imbalance. Fixed amplitude and phase imbalance compensation factors can, for example, be obtained as part of a factory tuning process. However, such phase imbalance compensation factors generally work adequately only under certain conditions.
Further, adaptive I/Q imbalance algorithms exist that use higher order statistics or other assumptions to estimate amplitude and phase imbalances in the RF signal. Such algorithms are, however, often complex and not sufficiently robust to changes in I and Q imbalance parameters.
Accordingly, there is a need for an improved VLIF receiver and a method of controlling a VLIF receiver.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.